1. Field of the Invention
The present invention relates to band pass filters, antenna duplexers used in high-frequency bands, and communication apparatuses incorporating the same.
2. Description of the Related Art
Dielectric filters, in which a plurality of resonant-line holes is aligned in a dielectric block, a resonant line being formed on the inner surface of each of the resonant-line holes, and an outer conductor being formed on the outer surface of the dielectric block, are disclosed in (1) Japanese Unexamined Patent Publication No. 6-310911, (2) Japanese Patent Publication No. (by PCT Application) 6-505608, (3) Japanese Unexamined Patent Publication No. 7-86807, (4) Japanese Unexamined Patent Publication No. 2-92001, and (5) Japanese Unexamined Patent Publication No. 5-37203.
In a dielectric filter described in (1), a hole is disposed passing through between a first end face and a second end face of the dielectric block, which are mutually opposing, a conductive film is formed on the surfaces except the first end face and in the through-hole, in which the first end-face side and the second end-face side of the through-hole have different sectional shapes, by which the characteristic impedance at the open-end side of the resonant line is different from that at the short-circuited-end side thereof to form a coupling between the resonators.
In a dielectric filter described in (2), a through-hole having a fixed sectional shape is formed in a dielectric block, in which a conductor is formed on the outer surfaces except one opening face of the through-hole and on the inner surface thereof, and an input/output terminal (a pad) is disposed on a side surface of the dielectric block.
In a dielectric filter described in (3), a through-hole having a conductive film formed on the inner surface thereof is disposed in a dielectric block, in which the conductive film on the inner surface of the through-hole and the conductive film on the outer surfaces of the dielectric block are electrically connected by one end face of the through-hole, whereas a recessed portion is formed on the other end face thereof, and a conductive film extended from the conductive film formed on the inner surface of the through-hole is formed on the inner surface of the recessed portion to form an additional capacitance at the opening end of the resonant line.
In a dielectric filter described in (4), similar to the case of (1), a hole is disposed passing through between a first end face and a second end face of a dielectric block, which are mutually opposing, and a conductive film is each formed on the surfaces except the first end face and in the through-hole, in which a step is arranged for dividing a large-diameter part and a small-diameter part of the through-hole, and adjacent resonant lines have differences in the inner-diameter ratios between the large-diameter parts and small-diameter parts thereof or in the axial-direction lengths of the small-diameter parts thereof.
In a dielectric filter described in (5), a through-hole and a groove whose bottom is an end face of the through-hole are formed in a dielectric block, in which a part of the dielectric block is cut off to widen the groove, and the conductor on the inner surface of the through-hole is extended to the conductor on the inner-side surface of the groove to produce a capacitance between the inner conductor and the outer conductor.
In each of the conventional dielectric filters disclosed in (1) to (3), the detailed structure of the through-hole passing through from the open face of the dielectric block to the short-circuited face thereof and the advantages obtained by the structure are not described. In the dielectric filter described in (4), in order to adjust the resonant frequencies of resonators so as to obtain a specified frequency balance, the inner-diameter ratio between the large-diameter part and small-diameter part of the resonant-line hole of each resonator or the axial-direction length of the small-diameter part thereof is changed with respect to an adjacent resonator (a resonant line) to relatively change the characteristic impedance of the open-face side part of the resonant line with respect to that of the short-circuited-face side part thereof so as to set a coupling coefficient between the adjacent resonant lines. In addition, in the dielectric filter described in (5), the part where a portion is cut off does not operate as a resonator and forms an additional capacitance.
In such conventional dielectric filters, it is difficult to make a coupling between specified adjacent resonant lines of the aligned plural resonant lines independently different from the coupling between the other resonant lines. Thus, there is a problem in that it is difficult to design a specified band-pass characteristic and attenuation-pole frequency. In order to obtain a specified filter characteristic, it is possible to use a method in which the pitch of the aligned resonant-line holes and positions (a stepped position) where the inner-diameters of the resonant-line holes are changed are each appropriately set. However, as a result, the size of the outline of the dielectric block is increased, which leads to an increase in the size of the overall dielectric block. Furthermore, when the sizes of the large-diameter part and small-diameter part of each of the resonant-line holes and the stepped positions of the resonant-line holes are not fixed, manufacturing efficiency is reduced.
To overcome the above described problems, preferred embodiments of the present invention provide a band pass filter, an antenna duplexer, and a communication apparatus incorporating the same, in which three or more resonant-line holes are aligned in a single dielectric block to easily obtain a desired filter characteristic and to achieve easy manufacturing.
One preferred embodiment of the present invention provides a band pass filter including three or more resonant-line holes disposed in a dielectric block, a resonant line formed on each inner surface of the holes, and an outer conductor formed on the outer surfaces of the dielectric block so that a dielectric filter is formed, wherein an end of the resonant line is a short-circuited end, and the other end thereof is an open end; the sectional area size of each of the resonant-line holes is changed at a certain point in the axial direction of the resonant-line hole; the sectional area size on the short-circuited-end side or open-end side of at least one of the plurality of resonant-line holes is made differently from those of the other resonant-line holes; and regarding the resonant-line holes symmetric with respect to a symmetry axis made at the center in a direction in which the plurality of resonant-line holes is aligned, the sectional area sizes of the short-circuited-end side or open-end side thereof are made the same so as to form coupling between adjacent resonant lines.
Another preferred embodiment of the present invention provides a band pass filter including three or more resonant-line holes disposed in a dielectric block, a resonant line formed on each inner surface of the holes and an outer conductor formed on the outer surfaces of the dielectric block to constitute a dielectric filter, wherein one end of the resonant line is a short-circuited end, and the other end thereof is an open end; the sectional area size of each of the resonant-line holes is changed at a certain point in the axial direction of the resonant-line hole; the distances between the short-circuited-end sides or open-end sides of the adjacent resonant-line holes are made asymmetrically with respect to an axis at the center in a direction in which the plurality of resonant-line holes is aligned so as to form coupling between adjacent resonant lines.
With the above described arrangements, the lengths of the resonant lines are fixed, and without changing the stepped positions of the resonant-line holes, a specified coupling between the adjacent resonant lines can independently be determined. Also, band-pass-filter characteristics can easily be obtained.
In the above described band pass filters, the sectional area sizes of the short-circuited-end side or open-end side of the resonant-line holes may be equal for all of the resonant-line holes.
In addition, in the above described band pass filters, the resonant-line holes may have the equally fixed lengths between the short-circuited ends and the points where the sectional area sizes are changed.
With the above described arrangements, when the dielectric block is molded, distribution of the internal stress is made uniform so that deformation and variation can be reduced. Furthermore, the structure of a metal die for molding the dielectric block is simplified and the production of the metal die is thereby easily performed, which leads to reduction in manufacturing cost.
Furthermore, in the above described band pass filters, an opening face of each resonant-line hole may be an open face where no outer conductor is formed and the open face may be used as the open end of the resonant line. With this arrangement, it is not necessary to dispose a coupling electrode on the open face of the dielectric block. Since the open end of each resonant line can be formed only by plane-processing of the open face, manufacturing cost can be reduced.
Furthermore, in the above described band pass filters, an nonconducive portion separated from the outer conductor may be disposed at a place recessed from the opening face of each resonant-line hole to make the nonconducive portion the open end of the resonant line. With this arrangement, since the open end of the resonant line exists inside the resonant-line hole, an electromagnetic leak is strongly reduced. In addition, since a position for disposing the nonconductive portion and the size thereof can be equal for all the resonant-line holes, it is not necessary to change processing conditions, with the result that shortening of the processing time and the reduction in manufacturing cost can be achieved.
Yet another preferred embodiment of the present invention provides an antenna duplexer including the dielectric filter having one of the above-described structures as a transmitting filter and a receiving filter formed in a single dielectric block, a transmitting-signal input terminal for being coupled to the initial-stage resonant line of the transmitting filter, a receiving-signal output terminal for being coupled to the final-stage resonant line of the receiving filter, and an antenna terminal for being coupled to the final-stage resonant line of the transmitting filter and the initial-stage resonant line of the receiving filter, respectively.
According to the above described arrangements, an overall compact antenna duplexer can be obtained.
Yet another preferred embodiment of the present invention provides a communication apparatus including the band pass filter or the antenna duplexer described above, which is disposed in a high-frequency circuit section.
According to the above described arrangements, a more compact communication apparatus can be obtained.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.